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or vertical directions, the spot of light on the aperture plate will have equal width on each side of the aperture and also above and below the opening, (Fig. V) ''c" should equal "d,"
If these settings are not correct, a larger spot is necessary to cover the aperture. For example, with the spot properly set it need be but ij^" in diameter to cover the aperture. This means a total area of 1. 2 1 square inches, of which 0.75 square inches of light goes through the aperture and 0.46 square inches, or 2>^% is wasted (Fig. VI. A). If the spot is but 1-8'' too low or too high, a i y-i6" diameter spot of light is necessary to cover the aperture, which means an area of 1.35 square nches, hence practically 50% of the available light flux is wasted (Fig. VI. B). A similar effect occurs if the spot is too far to either side.
Fig. 6
By means of other adjustments the lamp can be brought closer to or moved farther away from the condenser. To determine when the lamp is the correct distance from the condenser, place one wing of the rotating shutter in front of the objective lens. An image of the lamp filament will appear on this wing. This adjustment can be more satisfactorily made if the spot of light thrown by the mirror is not on the aperture plate in addition to that projected by the condenser. The mirror should be removed or turned to one side. By moving the lamp forward and back, a point will be found where the image is sharpest. This is the correct operating position of the lamp. The appearance of the spot on the shutter wing should be as shown in Fig. VII. A.
The accurate setting of the spherical mirror is also of great importance. The mirror performs two functions: It redirects that portion of the light from the lamp filament emitted backward and adds it to that projected directly on to the condenser. By so doing increases as high as 60% in screen illumination have been obtained. Secondly, the reflected light fills up the spaces between the filament coils and thus evens up the resultant screen illumination.
After placing the center of the reflector in a horizontal plane with the lamp filament, and the center of the condenser, the reflector should be moved forward and backward by means of adjustments provided for that purpose, until the reflected spot of light on the aperture plate is the same size as that projected by the condenser, in which position the filament image on the shutter will be of maximum sharpness. The mirror is then twisted or moved slightly sideways, until the dark
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